The t(8,21) is perhaps the most frequent chromosomal translocation associated with acute myeloid leukemia. This translocation fuses the DNA binding domain of AML1, to nearly all of ETO. ETO does not appear to bind DNA directly, but it interacts with co-repressors such as N-CoR, mSin3, and histone deacetylases-1, -2, and -3, and acts as a co-repressor by interacting with DNA binding proteins such as the PLZF transcriptional repressor Our preliminary studies indicate that AML1-ETO represses the endogenous promoter of the p14 ARF tumor suppressor. ARF is a component of the "oncogene checkpoint" and regulates p53 protein stability in response to oncogene activation. The levels of ARF are lower in t(8,21)-containing -ML samples and expression of AML1-ETO blocked oncogenic induction of ARF expression in primary murine myeloid progenitor cells. Based on this preliminary data, we hypothesize that AML1-ETO inactivates the oncogene checkpoint to "immortalize" HSC and myeloid progenitor cells, allowing the accumulation of further oncogenic mutations that cause AML. We have also identified the neurofibromatosis-1 (NF1) tumor suppressor as a second gene regulated by AML1-ETO. Leukemic blasts containing the t(8,21) have significantly lower levels of NF1 mRNA than other AML subgroups or the same subgroup (FAB M2) lacking the t(8,21). Therefore, we further hypothesize that repression of NF1 by AML1-ETO potentiates the oncogenic effects of secondary mutation of components of the RAS signaling pathway that are found in t(8,21)-containing AML. We propose to define the role of co-repressors that are recruited by AML1-ETO, and define the contribution of repression of the ARF and NF1 tumor suppressors to leukemogenesis.